Device for increasing the accuracy of addressing an electron beam striking a target

ABSTRACT

The present invention relates to systems which utilise a very thin electron beam (diameter one-tenth of a micron or less) to reach a point upon a given surface or to write-in an information there or to read one information therefrom. The object of the invention is to achieve a relative accuracy in the order of 10 5 to 10 6. Two scales are marked at two adjacent sides of a square or rectangular target. They comprise studs or spots of metal having a high secondary electron emission coefficient, these electrons being detected by an auxiliary electrode. These studs define a cross-ruled arrangement on the target, which serves for coarse guidance or location. Fine adjustment is obtained by interpolation within a small square of the cross-ruled arrangement.

tlnited States Patent 1191 Trotel DEVICE FOR INCREASING THE ACCURACY OFADDRESSING AN ELECTRON BEAM STRIKING A TARGET [75] Inventor: JacquesTrotel, Paris, France [73] Assignee: Thomson-CSF, Paris, France [22]Filed: Apr. 13, 1973 [21] Appl. No.: 350,719

[30] Foreign Application Priority Data Baldwin et al. 250/396 1451 Feb.4, 1975 Primary Examiner-T. H. Tubbesing Assistant Examiner-J. M.Potenza Attorney, Agent, or Firm-Oblon, Fisher, Spivak, McClelland &Maier [57] ABSTRACT The present invention relates to systems whichutilise a very thin electron beam (diameter one-tenth of a micron orless) to reach a point upon a given surface or to write-in aninformation there or to read one information therefrom. The object ofthe invention is to achieve a relative accuracy in the order of 10 to10*.

Two scales are marked at two adjacent sides of a square or rectangulartarget. They comprise studs or spots of metal having a high secondaryelectron emission coefficient, these electrons being detected by anauxiliary electrode. These studs define a cross-ruled arrangement on thetarget, which serves for coarse guidance or location. Fine adjustment isobtained by interpolation within a small square of the cross-ruledarrangement.

10 Claims, 6 Drawing Figures PATENTED 4W5 3.864.597

SHEET 3 OF 3 ADDER' ADDER 60 y 605 94-. y

INTEGRATOI} DEVICE FOR INCREASING THE ACCURACY OF ADDRESSING AN ELECTRONBEAM STRIKING A TARGET The present invention relates to systemscomprising an electron-gun and a target, located inside an evacuatedenclosure. The object of the invention is to provide a suitable devicein order to improve the accuracy of guidance and location of theelectron beam striking said target. The term addressing will be usedhere to describe the operation which makes it possible to attain a givenpoint on the target (guidance) and, to determine the coordinates of thepoint of impact of the electron beam. (location).

Those skilled in the art will be aware that in certain applications suchas the production of masks for integrated circuits and the execution ofwrite-in/read-out functions in relation to data stores, recourse is badto extremely thin electron beams of the order l/lOth to l/lOOth of amicron. For a scanned width of cm, there is thus a ratio of 2.10 to 2.10between the dimensions of the point and those of the scanned line. Anaccuracy of l/lOth of a micron in absolute value in a position of thepoint of impact, corresponds to a relative accuracy in the order of IO"to This kind of accuracy is incompatible with the unpredictabledeviations of the electron beam which occur under the influence ofparasitic magnetic fields around the cathode ray tube. To protect itagainst parasitic magnetic fields, a screening made of high-permeabilitymagnetic material, generally mu-metal, is used. However, a difficultystill remains: the accuracy of the deflection system must be improvedand, especially, the means for controlling the deflection must make itpossible to locate the point of impact with the desired accuracy.

The object of the present invention is an addressing device whichsatisfies this requirement.

The device in accordance with the invention comprises:

two levelling-rods which define two rectangular coordinate axes,perpendicular to one another and located in the plane of the target;

graduations carried by said target levelling rods and taking the form ofa substance which, under the impact of the electron beam, produces asecondary effect which differentiates said graduations from theremainder of the target and from those parts of said targetlevelling-rods which are located between said graduations;

means for detecting said secondary effect;

means for directing the beam, said beam successively impinging upon anelement of each levelling-rod.

The invention will be better understood and other of its featuresrendered apparent, from a consideration of the ensuing description andthe attached drawings in which:

FIG. 1 is a schematic section of an electronic masking apparatuscomprising an addressing mask-maker type, equipped with the device inaccordance with the invention.

FIG. 2 is a perspective view of the essential elements of the apparatusshown in FIG. 1.

FIGS. 3, 4 and 5 are explanatory diagrams pertaining to the operation ofsaid apparatus.

FIG. 6 is a block diagram of part of the device in accordance with theinvention.

In FIG. 1 a schematic section of an electronic masking apparatusincorporating an embodiment of the device in accordance with theinvention, has been shown. The masks concerned here are employed inplanar semiconductor technology and monolithic integrated circuits onsilicon substrates.

An evacuated enclosure comprises an inlet port 11, connected to apumping system which has not been shown, a bottom 12 with a bell-jar l3and inside the latter a hollow base 121 designed to support a target I.By way of example, the method of attachment of the target involves aflange 3 and springs 122 which load the target against the flange. Thetarget is generally constituted by a small plate which we will assume inthis case to be rectangularly shaped. Two levelling-rods 41 and 42 (thelatter being visible in FIG. 2 only) border two adjacent sides of thetarget.

The bell-jar 13 contains a tank 14 communicating with the outsidethrough an opening 141 and a tank 17 communicating with the outsidethrough an opening 171.

These tanks are designed to act as cryostats and to this end aresupplied respectively with liquid nitrogen 140 and liquid helium 170.The tank 14 surrounds the tank 17 which latter in turn surrounds acylindrical space in which the electron gun 5 containing cathode 7, islocated. This space is protected from external magnetic fields (in thecase of the invention) by a screen 19 which is rendered superconductivewhen the wall 18 against which it is located is at the temperature ofthe liquid helium. The screen 19 is extended at the end adjacent thetarget 1, in the form of a frustoconical portion 181 surrounding afurther frustoconical portion 16 which is itself the extension of thegun 5 and the cylinder 51 containing the focussing and deflectionelements of the cathode ray tube. The space 'between the screen 19 andthe frusto-conical portion 16, is narrow. This arrangement facilitatesmagnetic protection and the production of a high vacuum in theneighbourhood of the gun 5, because a cryogenic pumping effect isproduced along the wall 18 at the temperature of the liquid helium.

The gun 5 will preferably comprise an electron source 7 of the thinpoint field-excited cold-emission type, or again an electron source ofsome other type (the field effect benefits from the cryogenic pumpingaction). The electrons pass through an electrostatic or electromagneticlense 8 and then through two successive deflection systems 9 and 10.Also, above the levelling-rod system 41 and 42, and angle piece 20,having respective sides parallel to the rods and constituting anelectrode for picking up secondary electrons, is located.

In perspective, in FIG. 2, the electron beam 50 emitted by the source 7and passing successively through the lense 8, the devices 9 and 10 whichdeflect the beam so that for example it strikes the levelling-rod 41(direction OX of deflection), can be seen. These devices are deflectionunits which have been illustrated, by way of example,.in the form ofdouble pairs of deflection plates. The plates 91 and 101 are OYdeflection plates perpendicular to OX. They have been respectivelyconnected to the terminals and 105. Similarly, the plates 92 and 102 areOX deflection plates, respectively connected to the terminals 94 and104.

The deflection device 9, which has a low deflection amplitude isdesigned to improve the accuracy of addressing carried out by thedeflection device 10, the latter being the main, high-amplitudedeflector device. The device 9 is designed so that the positional errorin the point of impact of the beam upon either of the levelling-rods, isless than the width of a scale line and therefore much smaller than thegap between two scale lines. In the following, other conditions whichhave to be satisfied by the devices 9 and 10, will be described.

The levelling-rods 41 and 42 are connected in parallel to the terminal401 whilst the electrode 20 is connected to a terminal 402.

In the following: I, is the current flowing from the terminal 401 to thelevelling-rods, I, is the current through the electrode 20, flowingbetween it and the terminal 402; I; is the electron beam current. Takingaccount of the secondary emission effect, we have:

However, since in fact. at the terminals 401 and 402, we have therespective currents I and I,, we can express in these terms byrewritting equation (I), to give:

I 'l' I In FIG. 3, a part of the levelling-rod 41 has been shown, onwhich a rectangular graduation has been indicated in the form forexample of a gold stud or spot deposited upon a chromium substrate whichconstitutes the remainder of the levelling-rod. The zone of impact ofthe electron beam has been marked F. The aim is to locate the positionof the beam, in which the two aereas H and H of this zone of impacthaving the side of a graduation N, as their common limit, haverespective surfaces equal to one another.

Calling K, and K the secondary emission coefficients of chromium andgold, we have:

In the case where F only covers chromium; and

I, k I,

in the case where F only covers gold.

For the position in which H H we should obtain:

k (k k )/2 In FIG. 4, the variations in I, plotted as a function of thedistance 1 measured algebraically along an axis OZ parallel to thelevelling-rod 41 and having its origin at the left-hand edge of thegraduation N as indicated in FIG. 3 (Z: distance from the centre of F tothe axis 00), have been represented.

Taking the equations (2) and (6), at the point C where the graph of I,intersects the ordinates, we have:

This is the result which should be observed at the terminals 401 and 402at the instant of passage of the centre of the zone F across the edgeN,.

In fact, since it would be too difficult to manually verify the equation(7), recourse is had to a servo device which instantaneously measuresthe interval between the true position of F and the desired position (HH and translates the deviation into an error signal of such a sign that,through the deflector 9, it controls deflection of the beam 50 in adirection which shifts the beam 50 towards the desired position astridethe edge of a graduation.

By way of example, in FIG. 6 a single-wire diagram of a servo-devicecomprising the following elements, has been shown:

A comparator-detector 601;

two two-pole two-position (I and Il) switches 606 and two integrators602 and 603;

two adders 604 and 605, with controllable outputs.

It will be assumed first of all that the switches 606 and 607 are in theposition I. The device 10 is then supplied in such a fashion (voltage V,at the terminal 104, voltage V, at terminal that the beam 50 impingesupon the levelling-rod 41 in the neighbourhood of the graduation N Thecomparator-detector 601 supplies the terminals 401 and 402 with apositive direct voltage and, as we have seen, currents I and I, aregenerated as a consequence. The detected error signal is applied to theinput E of the adder 604 across the integrator 602. There are two otherinputs to the adder. the purpose of which will be explained hereinafter.The sole output of the adder 604 is connected to the terminal 94 (OXdeflection).

If we now assume that the switches 606 and 607 are both in the positionII, the system functions as if V, had been replaced by V E, by E and thelevelling-rod by the levelling rod 42.

Contrarily to that if the switch 606 is switched in position II and theswitch 607 in position I, the connections between the elements 601, 602and 603, are broken and the servomechanism disconnected.

The operation of the device will now be described by successivelyconsidering the two following problems:

first problem: guiding the beam and stopping it at the predeterminedpoint P on the target 1; second problem, locating the position of thebeam by measuring the coordinates of the point of impact. First problem:

In FIG. 5, a cross-ruled arrangement constructed on two rectangular axesOX and CY, has been shown which we will assume to be traced upon theplane of the target 1, this arrangement having been extended to containthe graduations on the levelling-rods 41 and 42, which are located alongthe axes OX and CY. Let Q, be a point having the coordinates N N suchthat for the point P (x, y), we have:

where x and y are less than unity (interval between two graduations oflevelling-rod).

The method of utilising the overall device consists in adjusting thebeam to the point Q (FIG. with a strict accuracy and then displacing thebeam with a lesser accuracy to obtain an impact upon the point P.

The method described hereinafter assumes that the following conditionsare satisfied:

a. Accuracy of control of the beam when shifted by the deflector 10,better than unity (interval between two scale lines);

b. Deflector has a scanning range which includes the target 1 and thelevelling rods 41 and 42;

c. Accuracy of control of the beam when shifted by the deflector 9,better than the thickness of a scale line;

d. Range of scanning of the beam when shifted by the deflector 9, atleast equal to unity;

e. Stability of the voltage supplies to the deflectors to be maintainedduring a time longer than a predetermined value, for example longer thanthat which is broadly required to execute a complete pattern in the caseof a production of a mask.

Control of the beam to bring it to the point O is carried out in twostages. First of all, the switches 606 and 607 (FIG. 6) are switched inposition I and the beam deflected in the OX direction. Then, theswitches 606 and 607 are switched in position II and the beam deflectedin the direction OY. Finally the servo-device is disconnected and thebeam displaced simply under the control of the device 9, by means of thevoltage u, and a applied respectively to the corresponding inputs of theadders 604 and 605. This presumes that the device 9 has been previouslycalibrated.

Second problem:

This method is the converse of the preceding one. The point of impact ofthe beam being given, but its coordinates being unknown, control isvaried until the beam strikes a point of known coordinates, for examplethe point 0,, and from this the coordinate of the point of impact aredetermined from the variation u, and u, in the voltages across theterminals of the device 9.

Distortions which may be exhibited by the device 10 must be taken intoaccount so that the addressing of the electron beam takes place in anidentical way from one apparatus to another. To this end, a start ismade by storing the corrections Au, and Au which have to be made to thevoltages u, and u in order to bring the point 0 (N N,,) to the point 0,,which is the intersection of standard cross-ruling arrangement appliedto the target 1. Calibration can be carried out, for example, byutilising the apparatus as a scanner-type electron microscope, in orderto show the standard crossruling arrangement.

The voltages 14,, u, and the corrections Au, and Au are introduced intothe servodevice of FIG. 6, at the auxiliary inputs of the adders 604 and605.

Finally, the drift in the supply voltages over a period of time must notbe neglected. The time T at the end of which this drift reaches the sameorder of magnitude as the resolving power of the apparatus, must bedetermined. The duration of the calibrating and addressing operationsshould be substantially less than the time T, if valid results are to beobtained.

By way of order of magnitude, a time of I second for the time T isindicated, and the operations can be carried out in less than 1 ms inthe case of the servodevice described hereinbefore.

The invention incorporates variant embodiments in which the graduationsof the levelling rods, under the effect of bombardment by the electronsof the beam are the location of secondary effects which differ from thesecondary emission phenomenon, for example:

the emission of back-diffused electrons;

the emission of X-rays;

the emission of light;

the creation of charge carriers in the material.

The invention applies to numerous cases where use is made of a very thinelectron beam which can be deflected in order to reach any point upon agiven surface, either to deposit information there or to read out apiece of information.

Amongst these applications, the following can be listed:

television tubes: the information furnished by the electron beamconstitutes an image which is rendered visible by a luminescent deposit;

television camera tubes: the information read by the beam is an opticalimage;

certain kinds of storage tubes for computers where the information isstored or read-out by electron beam respectively on or from appropriatedata carriers;

peripheral equipment of computers, which displays results ofcalculations upon the screen of a cathode ray tube;

mask-making machines for the manufacture of semiconductor devices wherethe resist of which the mask is made is exposed to a very thin electronbeam; certain machines for testing semiconductor devices in which theelectronic state of certain points of the circuit can be measured by theeffects produced there under the impact of an electron beam;

scanner-type electron microscope, microscopic sensing devices, etc.

What is claimed is:

1. A device for increasing the accuracy of addressing an electron beamstriking a surface, comprising in an evacuated enclosure a cathodecapable of emitting an electron beam in an electric field;

an electron beam deflection system;

an anode consisting of a target surface;

two levelling-rods perpendicular to one another said levelling rodsrespectively having graduations, said graduations being made of asubstance responding to electron impact, in another manner than that ofthe remainder of the rods; means for detecting the response to saidelectron impact; means for adressing a predetermined point of saidtarget, by scanning successively in the directions of one rod, and ofthe other, for impinging upon the corresponding graduation of the rods.

2. A device as claimed in claim I, wherein said response is secondaryemission of electrons.

3. A device as claimed in claim 1, wherein said deflection systemcomprises a servodevice which causes the beam to displace to a positionastride a predetermined edge of an arbitrary graduation on one or theother of said levelling-rods.

7 8 4. A device as claimed in claim 1, wherein said subval which istaken as unity; stance is gold, the remainder of said levelling-rods b.control of the deflection in order to bring the imbeing chromium. pactpoint of the beam to:

5. A device as claimed in claim 1, wherein said enclosure furthercomprises around said cathode a screen which is rendered superconductiveby thermic contact Y with a cryogenic source consisting of a reservoircontaining a liquid gas surrounding said screen.

6. A device as claimed in claim 1, wherein said dec. control of thedeflection in order to bring the impact point of the beam to:

flection means comprise a first and a second unit; said 10 X 0 firstunit serving to move the beam into the neighbour- Y N hood of apredetermined graduation, an said second unit serving to locate the beamastride one of the edges C t o f he eflection in Order to bring the imofsaid graduation. pact point the beam to the point:

7. A method of addressing an electron beam utilising N, a device asclaimed m claim 1, comprising the following stages: u

dennficanon predeiermmed e. control of the deflection in order toachieve the get surface, by its coordinates final point (X Y) X NJr x 8.A cathode ray tube comprising a device as claimed in claim I. Y y 9. Anelectron microsco e incor oratin a device as a I v p p g where N and N,,represent the number ofmtervals beclaimed in claim 1. tween graduationson each of levelling-rods arranged 10. An electronic mask-making devicecomprising a on the reference axes of the system (XX), and the device asclaimed in claim 1. quantities x and y represent fractions of such aninter-

1. A device for increasing the accuracy of addressing an electron beamstriking a surface, comprising in an evacuated enclosure a cathodecapable of emitting an electron beam in an electric field; an electronbeam deflection system; an anode consisting of a target surface; twolevelling-rods perpendicular to one another said levelling rodsrespectively having graduations, said graduations being made of asubstance responding to electron impact, in another manner than that ofthe remainder of the rods; means for detecting the response to saidelectron impact; means for adressing a predetermined point of saidtarget, by scanning successively in the directions of one rod, and ofthe other, for impinging upon the corresponding graduation of the rods.2. A device as claimed in claim 1, wherein said response is secondaryemission of electrons.
 3. A device as claimed in claim 1, wherein saiddeflection system comprises a servodevice which causes the beam todisplace to a position astride a predetermined edge of an arbitrarygraduation on one or the other of said levelling-rods.
 4. A device asclaimed in claim 1, wherein said substance is gold, the remainder ofsaid levelling-rods being chromium.
 5. A device as claimed in claim 1,wherein said enclosure further comprises around said cathode a screenwhich is rendered superconductive by thermic contact with a cryogenicsource consisting of a reservoir containing a liquid gas surroundingsaid screen.
 6. A device as claimed in claim 1, wherein said deflectionmeans comprise a first and a second unit; said first unit serving tomove the beam into the neighbourhood of a predetermined graduation, ansaid second unit serving to locate the beam astride one of the edges ofsaid graduation.
 7. A method of addressing an electron beam utilising adevice as claimed in claim 1, comprising the following stages: a.Identification of a predetermined point on said target surface, by itscoordinates X Nx + x Y Ny + y where Nx and Ny represent the number ofintervals between graduations on each of levelling-rods arranged on thereference axes of the system (X,Y), and the quantities x and y representfractions of such an interval which is taken as unity; b. control of thedeflection in order to bring the impact point of the beam to: Xo Nx Y Oc. control of the deflection in order to bring the impact point of thebeam to: X O Yo Ny d. control of the deflection in order to bring theimpact point the beam to the point: Xo Nx Yo Ny e. control of thedeflection in order to achieve the final point (X, Y).
 8. A cathode raytube comprising a device as claimed in claim
 1. 9. An electronmicroscope incorporating a device as claimed in claim
 1. 10. Anelectronic mask-making device comprising a device as claimed in claim 1.